gccrs: Remove fully the can_eq TyTy::BaseType::can_eq interface

This was an initial helper from back in the day which was not maintained
but was a nice const simple way to check if types are compatable. But
reusing our unify code is much much more acurate and single source of
truth on the type system.

gcc/rust/ChangeLog:

	* typecheck/rust-tyty.cc (InferType::can_eq): remove
	(ErrorType::can_eq): likewise
	(ADTType::can_eq): likewise
	(TupleType::can_eq): likewise
	(FnType::can_eq): likewise
	(FnPtr::can_eq): likewise
	(ClosureType::can_eq): likewise
	(ArrayType::can_eq): likewise
	(SliceType::can_eq): likewise
	(BoolType::can_eq): likewise
	(IntType::can_eq): likewise
	(UintType::can_eq): likewise
	(FloatType::can_eq): likewise
	(USizeType::can_eq): likewise
	(ISizeType::can_eq): likewise
	(CharType::can_eq): likewise
	(ReferenceType::can_eq): likewise
	(PointerType::can_eq): likewise
	(ParamType::can_eq): likewise
	(ConstType::can_eq): likewise
	(OpaqueType::can_eq): likewise
	(StrType::can_eq): likewise
	(NeverType::can_eq): likewise
	(PlaceholderType::can_eq): likewise
	(ProjectionType::can_eq): likewise
	(DynamicObjectType::can_eq): likewise
	* typecheck/rust-tyty.h: remove can_eq
	* typecheck/rust-tyty-cmp.h: Removed.

Signed-off-by: Philip Herron <[email protected]>

Author: philberty